(1) Field of the Invention
The invention relates to a process for the preparation, by spray-drying, of polyvinyl alcohols which are soluble in cold water and have been absorbed onto pigments.
(2) Description of the Related Art
Water-soluble, natural, semi-synthetic or fully synthetic polymers are employed in the preparation of industrial coatings, such as, for example, protective films for fabrics or pigmented paper coatings. Polymers, such as polyvinyl alcohols, containing polyhydroxyl groups are of particular importance among fully synthetic, water-soluble polymers. Polyvinyl alcohols are not only excellent binders for pigments for use in the preparation of extra white paper, but due to their carrier properties for optical brighteners, they surpass the water-soluble natural polymers, such as, for example, starch and the cellulose ethers prepared semi-synthetically, such as, for example, carboxymethylcellulose (see H. G. Oesterlin, Das Papier 36, 1982, pgs. 66-72, 121-126 and 170-175).
In polyvinyl alcohols, a distinction is made, depending on their degree of hydrolysis, between so-called fully hydrolyzed grades and partly hydrolysed grades (degree of hydrolysis 70-90 mol %). By virtue of their chemical structure, i.e., the hydroxyl group, strong intermolecular hydrogen bonds are formed, particularly in the case of the fully hydrolyzed polyvinyl alcohols, so that high solvation energies are required for dissolution in water (C. A. Finch, Polyvinyl Alcohol, John Wiley 1973, pgs. 22-25). With the exception of low-viscosity partly hydrolyzed polyvinyl alcohols, most of the polyvinyl alcohols employed in industry are, therefore, sparingly soluble in cold water. This constitutes a serious technical disadvantage, since the coating processes are encumbered by the added step of dissolving the polyvinyl alcohol, and additional energy costs associated therewith. Numerous dissolving techniques have already been suggested for improving the solubility of polyvinyl alcohols in cold water.
The solubility in cold water of polyvinyl alcohols can be improved by modifying them chemically, for example, reaction with glyoxylic or benzaldehyde carboxylic acids (German Patent No. 729,774) or with chloroacetic acid (U.S. Pat. No. 2,434,145) or with formaldehyde and sulfamic acid (German Patent No. DE-C 3,109,844).
In U.S. Pat. No. 4,013,805, the solubility in cold water is improved by the copolymerization of water-soluble comonomers, such as, for example, (meth)acrylic acid and maleic acid.
A variant which is of particular interest from the technical point of view for improving solubility in cold water is described in DE-A No. 3,622,820. By copolymerizing vinyl acetate with vinyl esters containing alkyl groups in the alpha-position, such as, for example, isopropenyl acetate, alkyl groups which, after the saponification to give polyvinyl alcohols, effect an interruption in the hydrogen bonds while maintaining the maximum OH number on the ethylene unit, are incorporated into the polymer chain. By this means, the improved solubility in cold water is combined with the pattern of properties of fully hydrolyzed polyvinyl alcohols.
Admittedly, an improvement in the solubility of polyvinyl alcohols in cold water can be obtained by the chemical measures described above. However, in practice, additional solution problems also occur because of the particle morphology of the polyvinyl alcohols prepared in this way.
In the industrial preparation of polyvinyl alcohols by the so-called "band process" or "kneader process" (Ullmann, Volume 19, pg. 376, 1980), unusual non-homogeneous polyvinyl alcohol compositions are obtained after the drying processes, and the particle size distribution of these has to be adjusted from approximately 150 to 750 .mu.m by time-consuming grinding and air classification processes to be suitable for good solubility in cold water. Major proportions of fine particles having particle sizes &lt;150 .mu.m result in agglomeration on contact with water, and major proportions of coarse particles having particle sizes &gt;750 .mu.m, result in extended dissolution times.
The grinding and screening processes rendered necessary by the non-homogeneous particle size distribution are not only technically involved, but also involve considerable safety risks as a result of the potential danger of dust explosions of the polyvinyl alcohol particles. Polyvinyl alcohol particles, as is known, contain residual amounts of methanol from the hydrolysis stage of their preparation. A further disadvantage of the frequently repeated grinding and screening processes for the preparation of particle morphologies soluble in cold water is that there is an increased incidence of metal attrition, which remains in the product and results in loss of quality during application.
It is, therefore, an object of the present invention to convert polyvinyl alcohols into a form soluble in cold water, and which does not have the disadvantages described above. The invention is further distinguished by the presence of methanol-free polyvinyl alcohol particles of uniform particle size distribution and very good solubility in cold water.
Spray-drying, in which aqueous solutions or suspensions of polymers are sprayed through nozzles by means of large quantities of air and are dried, is a technically simple process employed on a large industrial scale for the preparation of redispersible, water-soluble and water-insoluble polymer powders. However, this simple drying process by spray-drying does not give the desired result in the case of the polyvinyl alcohols soluble in cold water described above and in the case of chemically modified polyvinyl alcohols soluble in cold water, such as, for example, the alpha-alkyl substituted copolymers containing polyhydroxyl groups corresponding to DE-A No. 3,622,820. In spray-drying trials of 10% to 20% strength solutions of the abovementioned polymers, cobwebbing and considerable deposition on the walls of the dryers take place.
DE-A No. 2,200,322 relates to a process for the preparation of anhydrous polyvinyl alcohol/pigment preparations having an extremely high pigment content of 80% to 99.5% by weight. The aqueous polyvinyl alcohol/pigment suspension of this process is dried by means of shelf driers, belt conveyor driers, drum driers and fluidized bed driers, preferably drum driers. Admittedly, there is no risk of cobwebbing or wall deposition in this procedure because of the high proportion of pigment. However, due to the high proportion of pigment, the drying of the polyvinyl alcohol/pigment preparation is extremely energy-consuming and hence, uneconomical.
It has been found, surprisingly, that even when a substantially lower proportion of pigment is added to the solutions of the polyvinyl alcohols soluble in cold water, these solutions can be dried without cobwebbing and deposition on the walls.